Test: cassette input signal

[apoloval]

Mount the following schematic in a protoboard and test whether expected digital signal is successfully detected using a logic analyzer.

[apoloval]

There are several flaws in the design.

• The connectors are inverted. What was designed as CASIN output is actually the cassette input connector. And what was defined as cassete input connector is the CASIN output.
• The design is overcomplicated. There is no need for feedback, and some resistors are not useful in the design. This one is much simpler and should work as well.
  

Even though, I couldn't test none of both designs successfully. The LM311 parts I have are not actual LM311. They are labelled as so, but some other customers that purchased the same parts pointed out they are actually LM358 (basic op-amps). I ordered some more. I will continue with the tests once they arrive.

[apoloval]

Parts received: unblocked.

[apoloval]

Bingo! Original circuit, with a operative LM311.

[apoloval]

Note: 470K is too much for R35. That resistor have to raise the medium voltage of the input signal above reference voltage in IN+. As consequence, any minor noise in the line causes the LM311 to detect a drop below Vref, and a digital 1 to be output. A value of 220K raises the voltage a little bit more, and works perfectly. Noise doesn't affect then.

[apoloval]

Note: 10K is too much for R34. That resistor reduces the amplitude of the input signal. For data recorders, it's fine since they produce a signal of 5Vp-p. But, for regular cassettes (or PC outputs), that's too much since reduces the signal to 784mVp-p at max volume. Using a resistor of 4.7K gives a greater amplitude of 1.13Vp-p. Considering R35 is raising the voltage with some margin, a small amplitude could cause the squared output to be non-symmetrical. To avoid that, R34 could be set to 4.7K instead of 10K.

[apoloval]

The simplified circuit works fine. But the following amendments are needed:

• R35 is needed to raise the voltage over Vref, so the noise doesn't affect the output as stated in previous note. And its recommended value is 220K.
• R34 is needed in order to reduce the amplitude of input signal. Its recommended value is 4.7K.

That's the input for #40.